Add checks to prevent unauthorized relaying and impersonation

This patch adds checks that verify:

 - The envelope from must match the authenticated user. This prevents
   impersonation at the envelope level (while still allowing bounces, of
   course).
 - If the destination is remote, then the user must have completed
   authentication. This prevents unauthorized relaying.

The patch ends up adjusting quite a few tests, as they were not written
considering these restrictions so they have to be changed accordingly.

test/t-01-simple_local/run.sh

@@ -7,6 +7,7 @@ init
 
 generate_certs_for testserver
 
+mkdir -p .logs
 chasquid -v=2 --log_dir=.logs --config_dir=config &
 wait_until_ready 1025
 
@@ -16,6 +17,13 @@ wait_for_file .mail/someone@testserver
 
 mail_diff content .mail/someone@testserver
 
+# At least for now, we allow AUTH over the SMTP port to avoid unnecessary
+# complexity, so we expect it to work.
+if ! run_msmtp -a smtpport someone@testserver < content 2> /dev/null; then
+	echo "ERROR: failed auth on the SMTP port"
+	exit 1
+fi
+
 if run_msmtp -a baduser someone@testserver < content 2> /dev/null; then
 	echo "ERROR: successfully sent an email with a bad password"
 	exit 1